Method of driving transistor and shift register performing the same

ABSTRACT

A shift register has multiple stages each of which includes a pull-up part to generate a current gate line driving signal having a first state in response to a first control signal and a clock signal, a pull-down part to generate the current gate line driving signal having a second state in response to a second control signal, a pull-up driver to generate the first control signal to control the pull-up part in response to a previous gate line driving signal provided from a previous stage, a following gate line driving signal provided from a following stage, and an input voltage signal externally provided, and a pull-down driver to generate the second control signal to control the pull-down part in response to a third control signal provided from the pull-up driver and the input voltage signal, in which the second control signal swings between first and second voltage levels in association with the input voltage signal that swings between predetermined voltage levels.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to display devices for displaying images,and more particularly, to a shift register for the display devices and amethod of driving a transistor in the shift register.

2. Description of the Related Art

Generally, liquid crystal display devices are equipped with a gatedriver integrated circuit which is mounted on a liquid crystal displaypanel by means of a tape carrier package (TCP) or a chip-on-glass (COG)method. The liquid crystal display devices equipped with a gate driverintegrated circuit have disadvantages such as high the manufacturingcost and structural hindrance in designing a liquid crystal displaydevice. To overcome such disadvantages, the liquid crystal displaydevices have been developed to employ a “gate-IC-less” structure, inwhich no such gate driver integrated circuit is mounted on a liquidcrystal display panel. Instead, in the liquid crystal display deviceswith the “gate-IC-less” structure, a driving circuit usingamorphous-silicon thin film transistors (a-Si TFTs) is employed toperform the substantially same function as performed by the gate driverintegrated circuit.

Examples of a shift register circuit for display devices, which includesamorphous-silicon thin film transistors, are disclosed in the U.S. Pat.No. 5,517,542 and the U.S. Laid Open Publication No. 2002-0149318. Theshift register circuit disclosed in the U.S. Laid Open Publication No.2002-0149318 includes seven amorphous-silicon thin film transistors inits each stage.

FIG. 1 is a circuit diagram illustrating a stage of a conventional shiftregister such as one disclosed in the U.S. Laid Open Publication No.2002-0149318, and FIG. 2 is a block diagram illustrating a shiftregister having multiple stages. In a display device having such shiftregister, the shift register replaces the gate driver integratedcircuit. In other words, the shift register is integrated in a thin filmtransistor liquid crystal display panel to perform the same operation asthe gate driver integrated circuit does.

Referring to FIGS. 1 and 2, each of the stages in the shift registerincludes a pull-up part 110, a pull-down part 120, a pull-up driver 130and a pull-down driver 140. The present stage receives a gate linedriving signal GOUT_(N−1) (or scan line driving signal) from a previousstage, and the present stage generates a gate line driving signalGOUT_(N).

In case that the present stage is the first stage of the shift register,the first stage receives a start signal STV generated from a timingcontroller (not shown), and the first stage generates a first gate linedriving signal GOUT₁. In case that the present stage is the second stageof the shift register, the second stage receives the first gate linedriving signal GOUT₁ generated from the first stage, and the secondstage generates a second gate line driving signal GOUT₂. Also, in casethat the present stage is an Nth stage, the present stage receives an(N−1)th gate line driving signal GOUT_(N=1) generated from an (N−1)thstage, and the Nth stage generates an Nth gate line driving signalGOUT_(N). In like manner, the shift register having the N stagessequentially generates the gate line driving signals GOUT₁, GOUT₂, . . ., GOUT_(N).

The shift register also receives clock signals CKV and CKVB and voltagesignals externally provided, and each stage of the shift register hasmultiple input terminals to receive those signals as well as othercontrol signals and an output terminal to generate the correspondinggate line driving signal. A description of the overall operation of theshift register in FIG. 2 follows.

The first stage SRC₁ receives the start signal STV generated from thetiming controller (not shown), a gate turn-on voltage VON, a gateturn-off voltage VOFF, and a first clock signal CKV. The first stageSRC1 generates the first gate line driving signal GOUT₁ for selecting afirst gate line. The first gate line driving signal GOUT₁ is provided tothe first gate line and an input terminal (IN) of the second stage SRC2.

The second stage SRC₂ receives the first gate line driving signal GOUT₁generated from the first stage SRC1, the gate turn-on voltage VON, thegate turn-off voltage VOFF, and a second clock signal CKVB. The secondstage SRC2 generates the second gate line driving signal GOUT₂ forselecting a second gate line. The second gate line driving signal GOUT₂is provided to the second gate line and an input terminal (IN) of thethird stage SRC₃.

Likewise, the Nth stage SRC_(N) receives the (N−1)th gate line drivingsignal GOUT_(N−1) generated form the (N−1)th stage, the gate turn-onvoltage VON, the gate turn-off voltage VOFF, and the second clock signalCKVB. The Nth stage SRC_(N) generates the Nth gate line driving signalGOUT_(N) for selecting an Nth gate line. The Nth gate line drivingsignal GOUT_(N) is provided to the Nth gate line and an input terminal(IN) of the (N+1)th stage SRC_(N+1).

FIG. 3 is a timing diagram for describing the operation of theconventional shift register in FIGS. 1 and 2. Referring to FIGS. 1, 2and 3, the shift register receives the first clock signal CKV and thesecond clock signal CKVB and sequentially outputs the gate line drivingsignals to the gate lines formed on a TFT substrate. The second clocksignal CKVB has a 180° phase difference with respect to the first clocksignal CKV. The amplitudes of the first and second clock signals CKV,CKVB are in a range from about −8 volt to about 24 volt. The amplitudeof the output signal of the timing controller (not shown) is in a rangefrom about 0 volt to about 3 volt. Thus, the output signal of the timingcontroller (not shown) is amplified so that the first and second clocksignals CKV, CKVB have the amplitudes in the range from about −8 volt toabout 24 volt.

Since the NMOS transistor Q1 of the pull-up part 110 includesamorphous-silicon, the NMOS transistor Q1 has a relatively largetransistor size. This is because, in order to drive the liquid crystaldisplay device having a large screen size, a large amplitude of voltage(for example, from −14V to 20V) should be applied to the NMOS transistorQ1 due to the very small electron mobility of the amorphous-silicon ofthe NMOS transistor Q1. For example, in a liquid crystal display panelhaving a screen size of 12.1 inch (XGA), parasitic capacitance of a gateline has a value from about 250 pF to about 300 pF. Therefore, in orderto drive an amorphous-silicon thin film transistor designed inaccordance with minimum design rule 4 μm, a channel width of theamorphous-silicon thin film transistor should be about 5500 μm when achannel length of the amorphous-silicon thin film transistor is about 4μm.

Therefore, the parasitic capacitance between a gate electrode and adrain electrode of the NMOS amorphous-silicon thin film transistor Q1increases. The value of the parasitic capacitance is about 3 pF. Thisvalue causes a malfunction of the gate driver circuit employing the NMOSamorphous-silicon thin film transistor. The malfunction occurs asfollows.

The parasitic capacitor is electrically connected with a terminal towhich the clock signal CKV or CKVB having a large amplitude of voltage(for example, from about −14V to about 20V) is applied, and theparasitic capacitor (or coupling capacitor) is electrically connectedbetween the drain and gate electrodes of the NMOS amorphous-silicon thinfilm transistor Q1 to apply undesired voltage signal to the gateelectrode of the NMOS amorphous-silicon thin film transistor Q1.

Assuming that there is no means for maintaining the voltage level of thegate electrode of the NMOS amorphous-silicon thin film transistor Q1 atthe gate turn-off voltage level VOFF, the clock signal CKV or CKVBhaving the amplitude between about −14V and about 20V is applied to thegate electrode of the NMOS amorphous-silicon thin film transistor Q1. Inthis case, the voltage level of the gate electrode of the NMOStransistor Q1 becomes in the range from about −14V to about 20V, and theoutput signal equals to ‘20V (maximum value)−Vth (the threshold voltageof the NMOS amorphous-silicon transistor Q1)’. Applying such outputsignal to the gate line of the liquid crystal display panel causesabnormal display of images.

In order to maintain the voltage level of the gate electrode of thepull-up transistor Q1 at the gate turn-off voltage level VOFF, a holdtransistor Q5 is employed. The hold transistor Q5 is anamorphous-silicon thin film transistor. Also, a pull-down thin filmtransistor Q2 performing a pull down function is employed to maintainthe scan signal at the gate turn-off voltage level VOFF during most ofthe period after the pull-up transistor Q1 operates.

Since the a-Si transistor includes an N type MOSFET, the hold transistorQ5 receives a DC voltage signal proportional to the gate turn-on voltageVON (DC voltage signal) through the gate electrode of the holdtransistor Q5 during the period except for the time period of ‘onevertical synchronization period−two horizontal synchronization periods’.In addition, the pull-down transistor Q2 receives a DC voltage signalproportional to the gate turn-on voltage VON (DC voltage signal) throughthe gate electrode of the pull-down transistor Q2 during the periodexcept for the time period of ‘one vertical synchronization period−twohorizontal synchronization period’. Hereinafter, one verticalsynchronization period denotes a time interval between two consecutiveframes. Namely, one vertical synchronization period is referred to asthe time interval between the vertical synchronization signals (Vsync).The vertical synchronization signal (Vsync) indicates the point of timewhere a frame begins. One horizontal synchronization period denotes atime interval between two consecutive scan lines. Namely, one horizontalsynchronization period is referred to as the time interval between thehorizontal synchronization signals (Hsync). The horizontalsynchronization signal (Hsync) indicates the point of time where a scanline of a frame begins.

When the gate driver integrated circuit employs a-Si transistors, thepull-down transistor Q2 and the hold transistor Q5 may be deterioratedsince the DC voltage signal is applied to the gate electrodes of thepull-down transistor Q2 and the hold transistor Q5 for most of theoperation period.

When DC voltage signal is continuously applied to the gate electrodes ofthe a-Si transistors (pull-down transistor Q2 and hold transistor Q5)for a predetermined period, the pull-down transistor Q2 and holdtransistor Q5 are deteriorated, so that display quality of the liquidcrystal display device becomes deteriorated. In other words, since thethreshold voltages (Vth) of the a-Si transistors (pull-down transistorQ2 and hold transistor Q5) are increased due to the deterioration of thea-Si transistors, normal gate-source voltage Vgs becomes unable to turnon the a-Si transistors (pull-down transistor Q2 and hold transistor Q5)when the a-Si transistors (pull-down transistor Q2 and hold transistorQ5) have a predetermined threshold voltage Vth'.

FIG. 4 is a graph showing the variation of the threshold voltage of ana-Si TFT when a DC gate-source voltage is applied to the gate electrodeof the a-Si TFT. As shown in FIG. 4, when a DC gate-source voltage isapplied to the gate electrode of the a-Si TFT, the threshold voltage(Vth) of the a-Si transistor is increased due to the deterioration ofthe a-Si transistor. In case that the increased threshold voltage (Vth)reaches the DC gate-source voltage Vgs_dc, the a-Si transistor is notturned on even when a normal gate-source voltage Vgs is applied thereto.

SUMMARY OF THE INVENTION

The above disclosed and other drawbacks and deficiencies of the priorart are overcome or alleviated by a shift register and a method ofdriving a transistor according to the present invention. In oneembodiment, a shift register has a plurality of stages each providing agate line driving signal to a corresponding gate line of a displaypanel, and the stages each include a pull-up part to generate a currentgate line driving signal having a first state in response to a firstcontrol signal and a clock signal, a pull-down part to generate thecurrent gate line driving signal having a second state in response to asecond control signal, a pull-up driver to generate the first controlsignal to control the pull-up part in response to a previous gate linedriving signal provided from a previous stage, a following gate linedriving signal provided from a following stage, and an input voltagesignal externally provided, and a pull-down driver to generate thesecond control signal to control the pull-down part in response to athird control signal provided from the pull-up driver and the inputvoltage signal, in which the second control signal swings between firstand second voltage levels in association with the input voltage signalthat swings between predetermined voltage levels.

The pull-up part includes a pull-up transistor having a conduction pathbetween a terminal receiving the clock signal and a terminal generatingthe current gate line driving signal, and a gate electrode receiving thefirst control signal from the pull-up driver. The pull-up driverincludes a hold transistor to maintain a voltage level at the gateelectrode of the pull-up transistor at a selected voltage level. Thehold transistor has a gate electrode receiving the second control signalfrom the pull-down driver. An amplitude of a gate-source voltage of thehold transistor is larger than two times a threshold voltage of the holdtransistor. The input voltage signal has an amplitude larger than seventimes a threshold voltage of the hold transistor. The pull-down partincludes a pull-down transistor having a conduction path between aterminal generating the current gate line driving signal and a terminalhaving a selected voltage level. The pull-down transistor has a gateelectrode receiving the second control signal from the pull-down driver.

The pull-down driver includes an inverter to generate a fourth controlsignal in response to the input voltage signal and the third controlsignal from the pull-up driver, and a deterioration compensation part togenerate the second control signal in response to the input voltagesignal, the third control signal from the pull-up driver, and the fourthcontrol signal from the inverter. The inverter includes a firsttransistor having a conduction path between a terminal receiving theinput voltage signal and a first node, the first transistor operating asa diode, and a second transistor having a conduction path between thefirst node and a terminal having a selected voltage level and a gateelectrode receiving the third control signal from the pull-up driver, inwhich the inverter generates the fourth control signal from the firstnode. The deterioration compensation part includes a third transistorhaving a conduction path between the terminal receiving the inputvoltage signal and a second node and a gate electrode receiving thefourth control signal from the inverter, and a fourth transistor havinga conduction path between the second node and the terminal having theselected voltage level and a gate electrode receiving the third controlsignal from the pull-up driver, in which the deterioration compensationpart generates the second control signal from the second node.

In another embodiment, a method of driving a transistor having gate,drain and source electrodes includes applying a first voltage signal tothe drain electrode, applying a second voltage signal to the sourceelectrode, and applying a third voltage signal to the gate electrode tocontrol an electrical conduction path between the drain and sourceelectrodes, in which the third voltage signal swings betweenpredetermined voltage levels, so that a gate-source voltage signalestablished between the gate and source electrodes of the transistorswings between first and second voltage levels at a selected period. Thegate-source voltage signal has an amplitude larger than two times anormal threshold voltage of the transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention willbecome readily apparent by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a circuit diagram showing a stage of a conventional shiftregister;

FIG. 2 is a block diagram showing a conventional gate driver circuit;

FIG. 3 is a timing diagram for describing the operation of the stage inFIG. 1;

FIG. 4 is a graph showing the variation of the threshold voltage of ana-Si TFT when a DC gate-source voltage is applied to the gate electrodeof the a-Si TFT;

FIG. 5A is an equivalent circuit diagram of an a-Si TFT according to thepresent invention;

FIG. 5B is a graph showing a waveform of a gate-source voltage signalapplied to the gate electrode of the a-Si TFT in FIG. 5A;

FIG. 6 is a circuit diagram showing a stage of a shift registeraccording to the present invention;

FIG. 7 is a timing diagram showing a gate-source voltage signal appliedto the a-Si TFT;

FIG. 8 is a timing diagram showing another gate-source voltage signalapplied to the a-Si TFT;

FIG. 9 is a timing diagram showing further another gate-source voltagesignal applied to the a-Si TFT; and

FIG. 10 is a timing diagram showing still another gate-source voltagesignal applied to the a-Si TFT.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which exemplary embodimentsof the present invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein.

Hereinafter the preferred embodiment of the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 5A is an equivalent circuit diagram of the an a-Si TFT according tothe present invention, and FIG. 5B is a graph showing a waveform of agate-source voltage signal applied to the a-Si TFT. Referring to FIGS.5A and 5B, a drain voltage (Vd) is applied to a drain electrode (D) ofthe a-Si TFT, a source voltage (Vs) is applied to a source electrode (S)of the a-Si TFT, and a gate voltage (Vg) is applied to a gate electrode(G) of the a-Si TFT. When the gate voltage (Vg) is applied to the gateelectrode (G) of the a-Si TFT, the a-Si TFT becomes turned on or offdepending on the gate-source voltage (Vgs), a voltage difference betweenthe gate voltage (Vg) and the source voltage (Vs). For example, when thegate-source voltage (Vgs) applied to the gate electrode (G) of the a-SiTFT is lower than the threshold voltage of the a-Si TFT, the a-Si TFT isturned off. When the gate-source voltage (Vgs) applied to the gateelectrode (G) of the a-Si TFT is higher than the threshold voltage ofthe a-Si TFT, the a-Si TFT is turned on.

In this embodiment, a gate-source voltage (Vgs_ac) signal as shown inFIG. 5B is established between the gate electrode (G) and the sourceelectrode (S) of the a-Si TFT. The gate-source voltage (Vgs_ac) signalis a pulse signal swinging between a maximum voltage level Max(Vgs) anda minimum voltage level Min(Vgs) at a predetermined period. The gatevoltage (Vg) applied to the gate electrode (G) of the a-Si TFT is also apulse signal swinging between a maximum voltage level and a minimumvoltage level at a predetermined period, so that the gate-source voltage(Vgs_ac) is established between the gate and source electrodes of thea-Si TFT. By providing the gate-source voltage (Vgs_ac) in the a-Si TFT,the a-Si TFT operates normally even when the deterioration of the a-SiTFT occurs.

Since the gate-source voltage (Vgs_ac) between the gate electrode (G)and the source electrode (S) of the a-Si TFT swings between the maximumvoltage level Max(Vgs) and the minimum voltage level Min(Vgs) at thepredetermined period, the a-Si transistor is turned on even when anormal threshold voltage of the a-Si TFT is increased by ‘ΔVth’ due tothe deterioration of the a-Si transistor. As shown in FIG. 5B, since theincreased threshold voltage (ΔVth_ac) is smaller than the maximumvoltage level Max(Vgs), the a-Si transistor is turned on even when thenormal threshold voltage (Vtho) is increased by ‘ΔVth’ due to thedeterioration of the a-Si transistor. This description is presented bythe following Expression 1.Max(Vgs)−[Vtho+ΔVth]>0  Expression 1Here, ‘Vtho’ denotes the normal threshold voltage of the a-Sitransistor, ‘ΔVth’ denotes a voltage difference between the thresholdvoltage of the deteriorated a-Si TFT and the normal threshold voltage ofthe a-Si transistor. To provide the a-Si transistor with the gate-sourcevoltage (Vgs_ac), a pulse-type gate voltage (Vg) signal swinging betweena maximum voltage and a minimum voltage at the predetermined period isapplied to the gate electrode (G) of the a-Si TFT in a shift register.

FIG. 6 is a circuit diagram showing a stage of a shift registeraccording to an exemplary embodiment of the present invention. The shiftregister has multiple stages each having the substantially sameconfiguration as shown in FIG. 6. The stage of the shift registerincludes a pull-up part 210, a pull-down part 220, a pull-up driver 230,and a pull-down driver 240. The current stage receives a gate linedriving signal GOUT_(N−1) (or a scan line driving signal) from aprevious stage, and generates a current gate line driving signalGOUT_(N). When the current stage is a first stage of the shift register,the first stage receives a start signal generated fiom a timingcontroller (not shown), and generates a first gate line driving signalGOUT₁. When the current stage is a second stage of the shift register,the second stage receives the first gate line driving signal GOUT₁generated from the first stage and generates a second gate line drivingsignal GOUT₂. Likewise, when the current stage is an Nth stage, the Nthstage receives an (N−1)th gate line driving signal GOUT_(N−1) generatedfrom the (N−1)th stage and generates the Nth gate line driving signalGOUT_(N). The stages of the shift register are arranged in such a manneras to generate the gate line driving signals in sequence, and the shiftregister is integrated in a thin film transistor liquid crystal displaypanel.

The pull-up part 210 includes a NMOS pull-up transistor Q1. A clockterminal (CKV terminal or CKVB terminal) is connected to the drainelectrode of the NMOS pull-up transistor Q1, a gate electrode of theNMOS pull-up transistor Q1 is connected to a first node N1, and anoutput terminal OUT is connected to a source electrode of the NMOSpull-up transistor Q1.

The pull-down part 220 includes a NMOS pull-down transistor Q2. Theoutput terminal OUT is connected to a drain electrode of the NMOSpull-down transistor Q2, a gate electrode of the NMOS pull-downtransistor Q2 is connected to the pull-down driver 240, and a sourceelectrode of the NMOS pull-down transistor Q2 is connected to a gateturn-off voltage (VOFF) terminal.

The pull-up driver 230 includes a capacitor C and NMOS transistors (Q3,Q4, Q5). Particularly, the capacitor C is connected between the firstnode N1 and the output terminal OUT. A drain electrode of the NMOStransistor Q3 is connected to a gate turn-on voltage (VON) terminal, agate electrode of the NMOS transistor Q3 is connected to an outputterminal OUT_(N−1) of a previous stage, and a source electrode of theNMOS transistor Q3 is connected to the first node N1. For example, thegate turn-on voltage (VON) is a power voltage signal for the shiftregister. It is noted that the previous stage providing the previousgate line driving signal GOUT_(N−1) is a stage right next to the currentstage or one of the stages preceding the current stage.

The NMOS transistor Q4 of the pull-up driver 230 has a drain electrodeconnected to the first node N1, a gate electrode connected to a controlterminal CT, and a source electrode connected to the gate turn-offvoltage (VOFF) terminal. The gate turn-off voltage (VOFF) is, forexample, a power voltage signal for the shift register. The controlterminal CT receives a gate line driving signal from a following stage.It is noted that the following stage providing its output signal (i.e.,the gate line driving signal) to the current stage is a stage right nextto current stage or one of the stages following the current stage. Adrain electrode of the NMOS transistor Q5 is connected to the first nodeN1, a gate electrode of the NMOS transistor Q4 is connected to the gateelectrode of the pull-down NMOS transistor Q2, and a source electrode ofthe NMOS transistor Q5 is connected to the gate turn-off voltage (VOFF)terminal.

The pull-down driver 240 includes an inverter 242 and a deteriorationcompensation part 244. The inverter 242 includes two NMOS transistors Q6and Q7, and the deterioration compensation section 244 includes two NMOStransistors MA and MB.

In the inverter 242, gate and drain electrodes of the NMOS transistor Q6are commonly connected to the gate turn-on voltage (VON) terminal. Adrain of the NMOS transistor Q7 is connected to the source of the NMOStransistor Q6, a gate of the NMOS transistor Q7 is connected to thesource of the NMOS transistor Q3 via the first node N1, and a sourceelectrode of the NMOS transistor Q7 is connected to the gate turn-offvoltage (VOFF) terminal.

In the deterioration compensation part 244, a drain of the NMOStransistor MA is connected to the gate turn-on voltage (VON) terminal, agate of the NMOS transistor MA is connected to the source electrode ofthe NMOS transistor Q6 and the drain electrode of the NMOS transistorQ7. A source electrode of the NMOS transistor MA is connected to a thirdnode N3 that is connected to the gate electrodes of the pull-downtransistor Q2 and the hold transistor Q5. A drain of the NMOS transistorMB is connected to the third node N3. A gate of the NMOS transistor MBis connected to the first node N1, and a source electrode of the NMOStransistor MB is connected to the gate turn-off voltage (VOFF) terminal.The deterioration compensation section 244 provides a pulse-type voltagesignal to the gate electrodes of the pull-down transistor Q2 and thehold transistor Q5. The pulse-type voltage signal swings betweenpredetermined maximum and minimum voltage values.

Hereinafter, the operation of the stage in FIG. 6 is explained indetail. When a maximum voltage of the gate turn-on voltage signal VON isapplied to the gate turn-on voltage VON terminal, the NMOS transistor Q6is turned on and the gate capacitor of the NMOS transistor MA is chargedwith a voltage of the following Expression 2.V(MA_Gate)=Max(VON)−Vth(Q6)  Expression 2Here, ‘V(MA_Gate)’ denotes the voltage applied to the gate of the NMOStransistor MA, ‘Max(VON)’ denotes the maximum voltage of the gateturn-on voltage signal VON, and ‘Vth(Q6)’ denotes the threshold voltageof the NMOS transistor Q6. When the gate capacitor of the gate of theNMOS transistor MA is charged with the voltage V(MA_Gate), the thirdnode N3 connected with the gate electrodes of the pull-down transistorQ2 and the hold transistor Q5 has a voltage of the following Expression3.V(N3)=V(MA_Gate )−Vth(MA)=Max(VON)−Vth(Q6)−Vth(MA)  Expression 3Here, ‘V(N3)’ denotes the voltage at the third note N3, and ‘Vth(MA)’denotes the threshold voltage of the NMOS transistor MA.

When a minimum voltage of the turn-on voltage signal is applied to thegate turn-on voltage VON terminal, the NMOS transistor Q6 is turned offand the voltage applied to the gate capacitor of the NMOS transistor MAdecreases due to the coupling effect between the voltage level of thedrain electrode of the NMOS transistor MA and the voltage level of thethird node N3. Since the minimum voltage of the turn-on voltage signalis smaller than the voltage at the third node N3, the third node N3 isdischarged in accordance with the condition such as ‘V(MA_Gate)>Min(VON)+Vth(MA)’ (here, ‘Min(VON)’ denotes the minimum voltage of theturn-on voltage signal VON).

In case that the NMOS transistor MA is a symmetric transistor, half ofthe total parasitic capacitance of the NMOS transistor MA issubstantially equal to the parasitic capacitance Cgs between the gateand source electrodes of the NMOS transistor MA and also to theparasitic capacitance Cgd between the gate and drain electrodes of theNMOS transistor MA. In this case, when the minimum turn-on voltagesignal Min(VON) is applied to the gate turn-on voltage VON terminal, thethird node N3 has a voltage of the following Expression 4.V(N3)=Min(VON)+3×Vth(MA)  Expression 4This is because the gate electrode of the NMOS transistor MA isdischarged until the voltage V(MA_Gate) of the gate electrode of theNMOS transistor MA reaches the voltage value of ‘the minimum turn-onvoltage signal Min(VON)+the threshold voltage Vth(MA) of the NMOStransistor MA’. The voltage V(N3) at the third node N3 is applied to thegate electrodes of the pull-down transistor Q2 and the hold transistorQ5.

In this embodiment, to maintain the gate turn-off voltage level at thegate of the pull-up transistor Q1, the pull-down a-Si transistor Q2 isturned on when the voltage level of the clock signal (CKV or CKVB)changes from a low level to a high level during the period of ‘onevertical synchronization period−one horizontal synchronization period’.Also, the pull-down a-Si transistor Q5 is turned on when the voltagelevel of the clock signal (CKV or CKVB) changes from a low level to ahigh level during the period of ‘one vertical synchronization period−towhorizontal synchronization periods’.

In order to turn-on the pull-down transistor Q2 (or the hold transistorQ5), the maximum value of the gate-source voltage Vgs of the pull-downtransistor Q2 (or the hold transistor Q5) should be larger than the sumof the normal threshold voltage Vtho of the pull-down transistor Q2 (orthe hold transistor Q5) and the threshold voltage difference ΔVth. Thethreshold voltage difference ΔVth denotes the voltage difference betweenthe normal threshold voltage Vtho and the threshold voltage of adeteriorated pull-down transistor Q2 (or the hold transistor Q5). Thisis presented by the following Expression 5.Max(Vgs)−[Vtho+ΔVth]>0  Expression 5Here, ‘Max(Vgs)’ denotes the maximum value of the gate-source voltageVgs of the pull-down transistor Q2 (or the hold transistor Q5). Sincethe pull-down transistor Q2 or the hold transistor Q5 should bemaintained in a turn-on state even when the threshold voltage differenceΔVth reaches the value of ‘(Max(Vgs)+Min(Vgs))/2’, the followingExpression 6 should be satisfied.Max(Vgs)−[{Max(Vgs)+Min(Vgs)}/2+Vtho]>0Thus, {Max(Vgs)−Min(Vgs)}/2 >VthoTherefore, Max(Vgs)−Min(Vgs)>2×Vtho  Expression 6

According to the Expression 6, when the amplitude of the gate-sourcevoltage Vgs of the pull-down transistor Q2 (or the hold transistor Q5)is larger than two times that of the normal threshold voltage Vtho, thepull-down transistor Q2 (or the hold transistor Q5) normally operateseven when the pull-down transistor Q2 (or the hold transistor Q5) isdeteriorated.

The maximum value Max(Vgs) of the gate-source voltage Vgs and theminimum value Min(Vgs) of the gate-source voltage Vgs are presented bythe following Expressions 7 and 8, respectively.Max(Vgs)=Max(VON)−Vth(Q6)−Vth(MA)−VOFF  Expression 7Min(Vgs)=Min(VON)+3×Vth(MA)−VOFF  Expression 8

From the Expressions 6 to 8, the following Expression 9 is obtained.[Max(VON)−Vth(Q6)−Vth(MA)−VOFF]−[Min(VON)+3×Vth(MA)−VOFF]>2×Vtho  Expression9

In case that the threshold voltage Vth(Q6) of the NMOS transistor Q6 issubstantially equal to the threshold voltage Vth(MA) of the NMOStransistor MA and the threshold voltage Vth(MA) of the NMOS transistorMA is substantially equal to the normal threshold voltage Vtho of thepull-down transistor Q2 (or the hold transistor Q5), the followingExpression 10 is obtained from the Expression 9.Max(VON)−Min(VON)>7×Vtho  Expression 10

According to the Expression 10, when the amplitude of the gate turn-onvoltage VON is larger than seven times that of the normal thresholdvoltage Vtho of the pull-down transistor Q2 (or the hold transistor Q5),the pull-down transistor Q2 (or the hold transistor Q5) normallyoperates even when the pull-down transistor Q2 (or the hold transistorQ5) is deteriorated.

FIG. 7 is a timing diagram showing a waveform of the gate-source voltagesignal applied to the a-Si TFT, i.e., the pull-down transistor Q2 or thehold transistor Q5 in this embodiment. In the gate-source voltage signalin FIG. 7, for example, the pulse signal swings between a maximumvoltage level MAX(Vgs) and a minimum voltage level MIN(Vgs) and has apredetermined period substantially equal to the half of a period of theclock CK.

In this example, the rising edge of the gate-source voltage signal issynchronized with the rising edge of the clock signal CK, and thevoltage level of the gate-source voltage signal changes from the maximumvoltage level MAX(Vgs) to the minimum voltage level MIN(Vgs) at the half( 1/2H) of its period (H). Then, the voltage level of the gate-sourcevoltage signal changes from the minimum voltage level MIN(Vgs) to themaximum voltage level MAX(Vgs) when the voltage level of the clocksignal CK changes from a high level to a low level. Thus, thegate-source voltage Vgs has the rising edge synchronized with thetransition of the clock CK. In other words, the gate-source voltage Vgshas the maximum value when the voltage level of the clock CK changesfrom the low level to the high level or changes from the high level tothe low level.

The gate-source voltage Vgs applied to the pull-down transistor Q2 (orthe hold transistor Q5) has the amplitude larger than two times that ofthe normal threshold voltage Vtho of the pull-down transistor Q2 (or thehold transistor Q5).

FIG. 8 is a timing diagram showing a waveform of another example of thegate-source voltage signal applied to the a-Si TFT. Referring to FIG. 8,the gate-source voltage signal swings between the maximum voltage levelMAX(Vgs) and the minimum voltage level MIN(Vgs) has a predeterminedperiod substantially equal to the period of the clock CK.

In this example, the gate-source voltage signal is synchronized with theclock CK, such that the rising edge of the gate-source voltage signal issynchronized with the rising edge of the clock CK and the falling edgeof the gate-source voltage signal is synchronized with the falling edgeof the clock CK. The gate-source voltage signal of this example also hasthe amplitude larger than two times that of the normal threshold voltageVtho of the pull-down transistor Q2 (or the hold transistor Q5).

FIGS. 9 and 10 are timing diagrams showing further other examples of thegate-source voltage applied to the a-Si TFT. As shown in FIGS. 9 and 10,the phase of the gate-source voltage signal leads that of the clock CK.In FIG. 9, the phase of the rising edge of the gate-source voltagesignal leads the phase of the transition of the clock CK. In FIG. 10,the phase of the rising edge of the gate-source voltage signal leads thephase of the rising edge of the clock CK.

Although the exemplary embodiments of the present invention have beendescribed, it is understood that the present invention should not belimited to these preferred embodiments but various changes andmodifications can be made by one skilled in the art within the spiritand scope of the present invention as hereinafter claimed.

1. -17. (canceled)
 18. A method of driving a transistor having gate,drain and source electrodes, the method comprising: applying a firstvoltage signal to the drain electrode; applying a second voltage signalto the source electrode; and applying a third voltage signal to the gateelectrode to control an electrical conduction path between the drain andsource electrodes, wherein the third voltage signal swings betweenpredetermined voltage levels, so that a gate-source voltage signalestablished between the gate and source electrodes of the transistorswings between first and second voltage levels at a selected period. 19.The method of claim 18, wherein the gate-source voltage signal has anamplitude larger than two times a normal threshold voltage of thetransistor.